Add-on SDK: Reverse the tabs' order in the current window

gistfile1.js

const { ActionButton } = require("sdk/ui/button/action");
const { browserWindows } =  require("sdk/windows");

let button = ActionButton({
    id: "reverse-tabs",
    label: "Reverse Tab Order",
    icon: {
      // The new APIs use "./" as shortcut for data folder, you don't need
      // to require `self` for that purpose.
      "16": "./16.png",
      "32": "./32.png",
      "64": "./64.png"
    },
    onClick: function(state) {
      // Given the type of code you wrote, you don't want to reverse the tabs across all windows;
      // but only the tabs of the current one.
      let { tabs } = browserWindows.activeWindow;

      // console's method are already bound to the console object,
      // you don't need to do so.
      // Plus, by default, `console.log` dumps only when you're in
      // development mode (from cfx run) and not when you install
      // the add-on (XPI); so you don't need an additional variable `debug`.
      let LOG = console.log;
 
      // don't use for...each, is deprecated.
      // See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for_each...in
      let notPinned = ({isPinned}) => !isPinned;
      let byIndex = ({index: a}, {index: b}) => a - b;

      // We need to get the tabs ordered by index, 'cause tabs
      // module returns the tabs always in the same order
      let normalTabs = Array.filter(tabs, notPinned).sort(byIndex)

      let { length } = normalTabs;

      // we iterate only half tabs, because we switching them
      let count = length >> 1;

      for (let i = 0; i < count; i++) {
        let index = normalTabs[i].index;
        let k = length - i - 1;
        normalTabs[i].index = normalTabs[k].index;
        normalTabs[k].index = index;
      }

    }
});